Why, because sexes are not different from each other?
What's immoral is to state things that exist differently than what they really are, as if you know.
The real problem is most people don't have the understanding (or, otherwise, safe conditions) to accurately state the differences, but they clearly exist. Otherwise none of us would be here.
The problem lies in the ambiguity between prescriptive and descriptive statements.
If you describe the average woman whom you have interacted with as being more kind and nurturing, that's totally reasonable. You're just being descriptive.
If you prophesy that the average woman that exists will be more kind and nurturing, then you are making an assertion without enough data. There are plenty of women who don't fit that generalization.
If you proport that women in general are inherently better equipped to be in a nurturing role, then you are being prescriptive. That implies that women should be more nurturing, and that those who are more nurturing should be more feminine. That's sexism framed as benevolence.
"Benevolent" in this case is defining the category, not the morality of the sexism.
Indeed, sexism is inherently immoral, even when it is framed as benevolence.